This invention relates to a heap bioleaching process which is operated at elevated temperatures to achieve a high rate of mineral oxidation.
The invention is described hereinafter with particular reference to the leaching of chalcopyrite ore for the recovery of copper. This is by way of example only and, where relevant, the principles of the invention can be used for the leaching of other ores for the recovery of metals.
A heap containing chalcopyrite ore can be leached effectively if the heap temperature is in the thermophilic region i.e. above 60° C. and preferably from 65° C. to 70° C.
When a biological leaching process is started on a heap the temperature of the heap is initially at ambient temperature. Energy which is generated by the activity of suitable microorganisms which are introduced into the heap or which occur naturally gradually increases the heap temperature. However, the leaching activity of the microorganisms tails off radically in the temperature range of 50° C. to 60° C. and the heap temperature cannot readily rise above 60° C., a temperature level at which thermophilic cultures are activated. This phenomenon severely reduces the effectiveness of a biological leaching process carried out on chalcopyrite.
FIG. 1 of the accompanying drawings illustrates on a background of temperature versus time groups of microorganisms which are operative in different temperature regions. At normal mesophilic conditions the dissolution of chalcopyrite is very poor. Specific microorganisms grow in the higher temperature region and these microorganisms are critical to maintain a high Eh environment at elevated temperatures for chalcopyrite leaching.
FIG. 2 includes curves, marked AT, AC, AF and SM to designate Acidithiobacillus thiooxidans, Acidithiobacillus caldus, Acidithiobacillus ferrooxidans and Sulfolobus metallicus, respectively, which represent growth or activity rates for these microorganisms as a function of temperature. Microorganisms which are able to grow in the mesophilic region (up to 40° C.) die when the temperatures are increased to moderate thermophilic temperatures (50° C. to 60° C.). Similarly the moderate thermophilic microbes are not able to survive under thermophilic temperatures (in excess of 60° C.) and only the thermophilic microbes are able to grow in this temperature region. It is important therefore that a transition takes place from active mesophiles to active moderate thermophiles and then to active thermophiles in a heap leaching environment as the temperature rises inside the heap. If one of the microbial groups is absent microbial succession cannot take place successfully and thermophilic conditions cannot be reached.
FIG. 3 illustrates a number of curves of heat or temperature variation, as a function of time, obtained in a simulated heap leaching environment. An air stream AS is directed into a heap to deliver oxygen and carbon dioxide to the microorganisms. Although the air stream is required it does exhibit a cooling effect on the heap and, in order to conserve heat, the air flow rate must be decreased.
A liquid stream LS of raffinate is drained from the heap. Heat extraction via the raffinate increases with the raffinate flow rate and, again, to conserve heat at a high reaction rate, the raffinate flow rate must be reduced.
The heat which is generated (HG) by the oxidising microorganisms increases as the reaction rate increases.
A curve AH reflects the accumulated heat in the heap, while the average temperature in the heap is reflected by a curve marked AT.
Four time zones 1 to 4 are marked in FIG. 3. In zone 2 the heap temperature has a significant dip. The temperature thereafter increases (zone 3) although, in zone 4, the temperature again decreases significantly. In zone 1, where the heat generation HG surpasses the heat losses AH, the heap temperature increases rapidly. This results in an increase in the heap temperature as pyrite oxidation increases.
It is evident from the aforegoing that a significant problem exists in bioleaching a heap of chalcopyrite ore in that the temperature gap of 50° C. to 60° C. in the heap must be carefully bridged to ensure that the heap temperature reaches the thermophilic zone at which chalcopyrite is amenable to effective bioleaching.
It is an object of the present invention to provide a method of operating a heap bioleaching process which addresses, at least partly, the aforementioned aspects.